1. Field of the Invention
The present invention relates to a dental handpiece having therein non-contact rotational speed detection device.
2. Prior Art
For dental treatment, a tool tip on the head of a handpiece is rotated at a high speed. However, when cutting load increases, rotating speed is usually reduced significantly. To know the rotating speed under load is desirable for proper treatment, useful for data analysis of clinical experiments, and necessary to control the rotating speed at a constant value. For these purposes, prior art attempts to detect the rotating speed of the motor for a micro-motor driven handpiece were based upon motor drive voltage or current. However, this attempt has not been successful since the rotating speed is detected indirectly via voltage or current, and thus large errors are caused. The magnetic speed detection system, which was recently developed by the applicant and has been used for pneumatic handpieces, includes a magnetic resistance device or a Hall effect element as a detection means and generates a pulse voltage using a magnetic resistance effect caused by the rotation of the rotor, a part of which is magnetized or in which a small permanent magnet piece is imbedded. In the system, the rotor is very small, and incorporates a chuck mechanism to hold a cutting tool. However, it is very difficult to imbed a magnet piece in the rotor. When magnetizing the rotor, the material of the rotor must be specially selected. Furthermore, bearings made of a non-magnetic material must be used to easily detect changes in the magnetic flux; therefore, bearings made of ordinary magnetic metal cannot be used. This is a fatal defect of the magnetic speed detection device.